The drawing of metals into a variety of shapes is a well-known metal forming process. These shapes include cylindrical cups and tubes with curved side walls as well as shapes with angular side walls, with square or rectangular cross sections, for example. Countless numbers of items are produced by this process, with one example being a grenade body. Typical metals used in the process are carbon steel, alloy steel, aluminum, and brass, as well as other types of metals.
A common shape desired to be formed by drawing is essentially a cylindrical cup formed with one end closed. The cup may be drawn in a single or multistage process. Each stage includes a punch which drives the metal to be formed into a die to form an intermediate or final shape. In the typical multi-stage process, the metal is processed through a number of draw stations and completed in a series of finishing stations. The number of draw stations required depends upon the inside diameter of the cylinder, the height of the cylinder, metal thickness and physical properties of the metal.
Previously known punch and die forming machines are adequate to form cup shapes when the desired end configuration does not need to be sharply defined with very close dimensional tolerances. With thicker materials, the prior known processes are not adequate. With such thick material, the punch is pressing against a small cross section at the bottom of the drawn part while pulling the part through the die. This imposes a tensile stress in the cylindrical portion of the cup. If the tensile stress in the cylindrical portion exceeds the ultimate tensile strength of the material, the bottom of the cup will separate from the cylinder, thereby resulting in a defective part. Even though ultimate failure may not occur, excessive thinning of portions of the cup can cause cracks and splits to occur.
A prior attempt to eliminate problems in drawing is disclosed in U.S. Pat. No. 4,147,049 issued to Book et al. on April 3, 1979. This patent discloses the use of supplemental sleeves which assist a punch in drawing a cup into a die by contacting the open end of the cylindrical cup to reduce the tensile stress in the cylindrical portion of the cup. However, with such a prior technique, the open end of the cylindrical cup drawn does not always remain perfectly square with the axis of the cylinder. Depending on the properties of the metal drawn and the ratio of length to diameter of the drawn part, the open end may have an irregular or wavy surface of variable severity so that the supplemental sleeves do not provide a uniform compensating stress within the cylindrical portion of the cup. The height of these irregularities varies from part to part and it is therefore impossible to apply a constant force on each part.
A need therefore exists to overcome the above recorded problems in drawing metal. In particular, a need exists to reduce the tensile stress in the side wall portions of a drawn piece to permit precisely controlled shaping held to extremely close tolerances and even permit changes in thickness of metal within a closed end of the piece.